The 39th Infantry Regiment is a parent regiment in the United States Army. Originally organized for service in World War I, the 39th fought in most of the conflicts involving the United States during the 20th century, and since 1990 the 2nd Battalion has served as a training unit stationed at Fort Jackson, South Carolina. The 3rd Battalion was started on 21 October 2015 and a 4th Battalion was added in July 2017.